The plain film criteria for a small bowel obstruction follows the rule of
3's: small bowel dilated to 3 cm, greater than 3 air-fluid levels, or a
small bowel wall greater than 3 mm thick. However, the routine KUB is
neither sensitive nor specific for obstruction and many patients in the ER will
have a "non specific" bowel gas pattern requiring followup with either clinical
exam, repeat KUB, or CT of the abdomen. In our ER most patients with suspected bowel pathology go on to a CT examination. Many do not start
with a KUB. This is probably OK. It is easier to identify
obstruction on CT and it is often possible to identify the transition point.
The KUB remains useful as a quick check for free intraperitoneal air in an
acutely ill patient though many attendings in our ED have begun ordering non
contrast CT scans to evaluate for bowel perforation.

A patient with SBO will have dilated small bowel proximal to the obstruction
and relatively decompressed small bowel distal to the obstruction. We use
3cm as the maximum diameter for normal small bowel. However, common sense
must prevail. If
the proximal bowel is 2.8 cm and the distal bowel is completely collapsed there
is an obstruction. The dilated bowel can be
completely fluid filled,
or have multiple air fluid levels. The distal bowel does not
need to be completely collapsed. In a complete obstruction the distal
bowel will eventually collapse. However, depending on the severity of
the obstruction and the timing of the scan, air and fluid can still be seen
distally. For this reason, a patient with SBO may still pass flatus and
can have continued bowel movements. A scan with dilated proximal bowel and
incompletely collapsed distal bowel is indicative of
an early or partial obstruction.

When looking at a CT to assess for SBO, I first look for two things:
dilated proximal bowel and decompressed distal bowel.
Look in the right
lower quadrant for the decompressed bowel. This is where the distal
portion of the small bowel is located. Once you see these two
things you know there is an obstruction. Next look for a transition point
by following the bowel backward from the cecum and forward from
the duodenum. If the obstruction is distal, it is easier to work backward
from the cecum. If the obstruction if proximal, it is easier to work
forward from the duodenum. The transition point is a focal area where the caliber of the
bowel abruptly changes. In this case, if you follow the bowel backwards from the cecum
you can easily see the transition point in a
right femoral hernia.

Causes of hernia include adhesions from prior surgery, hernia, intra-abdominal mass
(which can be either extrinsic or intrinsic to the bowel),
stricture from inflammatory bowel disease, or
edema from recent bowel surgery or inflammatory bowel disease. Adhesions
are the most common cause, and while the transition point may be easily seen,
the adhesion itself is often not identified.

If you cannot find the exact
location of the transition point that is OK. You have identified the
patient has a SBO
and treatment can begin. The initial treatment for SBO is nasogastric tube
decompression, IV fluid hydration, and serial physical exams. However,
several imaging features of SBO can push the patient to the operating room and
you should be aware of what they are.

Is there small bowel wall
thickening or focal areas of mesenteric stranding? Distended bowel
should have a paper thin wall. Edema in a distended wall suggests
ischemia. Pneumatosis in distended loops of small bowel suggests
necrosis. Check to make sure there is no air in the SMV or portal vein. In this example there is stranding in the mesenteric fat
within the
hernia. This suggests that the hernia is strangulated and at risk of
ischemia or necrosis.

Is there ascites? If the
patient has cirrhosis, low albumin, CHF, or other causes for third spacing
fluid then the ascites may be unrelated. However, if there is no good explanation for
the ascites the patient may have ischemia secondary to the obstruction.

Is there a
closed loop
obstruction? In a closed loop obstruction you will see a loop of
distended bowel that loops back onto itself. The loop itself will be
distended. The bowel just proximal to the loop will also be
dilated. The bowel distal to the loop will be decompressed. The
transition point is located where the bowel crosses itself. This can be
caused by an adhesion, an internal hernia, or a volvulus (twisting of the
bowel). Closed loop obstructions are more likely to be treated with surgery
because of the increased rate of perforation. The loop
cannot drain its internal secretions and is prone to rupture.

Is there free air? This
means the patient has a perforated viscus and probably needs an operation.

Some important caveats to remember.

Oral contrast does not have to progress to the
transition point. It frequently will not because of fluid and air
backs up in the obstructed bowel. If the proximal loops are dilated
and the distal loops are collapsed, call it SBO. You do not need to
re-scan to check the passage of contrast unless you are unsure if the proximal
loops are really different in caliber from the distal loops.

If you think there is a caliber change in the
bowel but contrast is seen distal to this area then the patient does not have
a complete SBO. They may have a partial or intermittent obstruction and
you should tell the surgeons this. Also, ask if the patient has had a
recent scan with oral or rectal contrast.. You may be seeing barium from a
prior study.

Followup scans can be misleading. If the
patient is being decompressed via nasogastric tube then the proximal small
bowel will look less dilated. This can make a complete obstruction
look like a partial obstruction because it appears as if some of the fluid is
passing past the transition point. Double check the end of the contrast
column. If it is still proximal to the transition then there is no real
evidence of resolution. You can also ask the surgeon to correlate the
results of the scan with the amount of NGT output. A large volume of
aspirate will explain the interval decrease in size of the proximal small
bowel.